Electric welded boiler steel pipe

ABSTRACT

This invention provides a boiler steel pipe that exhibits a high creep rupture strength on a high-temperature high-pressure side and is excellent in electric weldability, and an electric welded boiler steel pipe having fewer defects at an electric welded portion. The boiler steel contains, in terms of wt %, C: 0.01 to 0.20%, Si: 0.01 to 1.0% and Mn: 0.10 to 2.0%, contains further Cr: 0.5 to 3.5%, and limits p≦0.030%, S≦0.010% and 0≦0.20%, wherein a weight ratio of (Si %)/(Mn %) or (Si %)/(Mn %+Cr %) is from 0.005 to 1.5, the balance Fe and unavoidable impurities, and the melting point of the mixed oxide of SiO 2  and MnO, or SiO 2 , MnO and Cr 2 O 3 , is not higher than 1,600° C. The oxide that would otherwise result in the defects of the electric welded portion is molten and squeezed out as slag components. Therefore, a boiler steel excellent in electric weldability and the electric welded boiler steel pipe having fewer welding defects, excellent in creep rupture strength and toughness, and using the former, can be obtained.

TECHNICAL FIELD

This invention relates to steel for a boiler and an electric weldedboiler steel pipe using the boiler steel. More particularly, thisinvention relates to steel, for use in a high-temperature/high-pressureenvironment, that is excellent in creep rupture strength and electricweldability, and an electro-unite boiler steel pipe that has excellentproperties at the electrically welded portions.

BACKGROUND ART

An austenite type stainless steel, a high Cr ferrite steel having a Crcontent of 9 to 12% (the term “%” means “% by weight”; hereinafter thesame), a low Cr ferrite steel having a Cr content of not greater than2.25% or a carbon steel has been generally used for high-temperature-and high-pressure-resistant members for boilers and for chemicalindustry and nuclear facilities. These steels are selected appropriatelyin consideration of the environment of use of the members such as thetemperature, the pressure, etc, and economy.

Among these materials, a low Cr ferrite steel having the Cr content ofnot greater than 2.25% has the following features. Since this steelcontains Cr, it is superior to carbon steel in oxidation resistance,high-temperature corrosion resistance and high-temperature strength. Alow Cr ferrite steel is far more economical than an austenite typestainless steel. It has a small coefficient of thermal expansion anddoes not undergo stress corrosion cracking. It is also more economicaland more excellent in toughness, heat conductivity and weldability thana high Cr ferrite steel.

Typical examples of such a low Cr ferrite steel are STBA20, STBA22,STBA23, STBA24, etc, that are stipulated by JIS. These low Cr ferritesteels are ordinarily called generically “Cr—Mo steels”. The low Crferrite steels, to which V, Nb, Ti, Ta or B is added as a precipitationhardening element to improve the high-temperature strength, are proposedin Japanese Unexamined Patent Publication (Kokai) Nos. 57-131349,57-131350, 61-166916, 62-54062, 63-18038, 63-62848, 1-68451, 1-29853,3-64428, 3-87332, and so forth.

A 1Cr-1Mo-0.25V steel as a turbine material and a 2.25Cr-1Mo—Nb steel asa structural material of a fast breeder reactor are well known as theprecipitation hardening type low Cr ferrite steel. However, these low Crferrite steels are inferior to the high Cr ferrite steel and theaustenite type stainless steel in the oxidation resistance and thecorrosion resistance at high temperatures, and have lowerhigh-temperature strength. Therefore, these steels involve the problemswhen used at a temperature higher than 550° C.

To improve the creep strength at a temperature of 550° C. or above,Japanese Unexamined Patent Publications (Kokai) No. 2-217438 and No.2-217439 propose low Cr ferrite steels to which large amounts of W areadded or Cu and Mg are added compositely. Japanese Unexamined PatentPublication (Kokai) No. 4-268040 proposes low Cr ferrite steel to whicha trace amount of B is added after limiting the N content in order toimprove the creep strength at a temperature of 550° C. or above and torestrict the drop of toughness resulting from the increase of thestrength.

When these materials are electrically welded, a large number ofhigh-melting-point oxides are formed at the electric welded portion andare entrapped into the inner surface at the time of electric welding.Consequently, a defect area ratio of the electric welded portion, as oneof the properties of the electric welded portion, is high, and theproperties of the electric welded portion, such as the creep rupturestrength, toughness, etc., cannot be satisfied in a high-temperatureenvironment of 550° C. or above. Therefore, these materials cannot besaid to be suitable materials for electric welded steel pipes. For thesereasons, the low Cr ferrite steel which is capable to use at atemperature of 550° C. or above can be nominated a seamless steel pipe.However, the production cost of the seamless steel pipe is high, andthis material is not a useful material from the aspect of economy.

In view of the technical background described above, it is an object ofthe present invention to provide a steel for a boiler that is anordinary steel not containing Cr (ordinary boiler steel) and a low Crferrite steel having a Cr content of not greater than 3.5% (low Crferrite type boiler steel), exhibits a high creep rupture strength afteruse at a high temperature for a long time, is particularly excellent inelectric weldability with fewer defects formed at an electric weldedportion, and an electric welded boiler steel pipe having fewer defectsat the electric welded portion and produced by using the steel.

DISCLOSURE OF THE INVENTION

The present invention relates to an electric welded boiler steel pipethat can be used at a temperature of 550° C. or above, can be producedat a lower cost of production but has a better economical effect thanconventional seamless steel pipes.

The inventors of the present invention have conducted intensive studiesto obtain a steel and a steel pipe having fewer defects generated at anelectric welded portion and having better properties, such as creeprupture strength and toughness, then in ordinary boiler steels and lowCr ferrite type boiler steels. As a result, the present inventors havefound that a binary system mixed oxide of SiO₂ and MnO formed at thetime of electric welding exerts a great influence on the welding defectsin ordinary boiler steels, and a ternary system mixed oxide of SiO₂, MnOand Cr₂O₃ exerts a great influence on the occurrence of the weldingdefects in low Cr ferrite type boiler steels. The present inventors haveclarified further that when the melting points of the respective mixedoxides are lowered, the oxides are molten at the time of electricwelding and can be squeezed out as slag components from the weldportion, and this reduces the welding defects of the electric weldedportion resulting from the mixed oxides.

The present invention was completed on the basis of the findingdescribed above. As to the ordinary boiler steels, the relationalformula of Si and Mn is derived on the basis of the binary system phasediagram, and the respective contents are stipulated to lower the meltingpoint of the binary system mixed oxide of SiO₂ and MnO. As to the low Crferrite type boiler steels, the relational formula of Si, Mn and Cr isderived on the basis of the ternary system phase diagram of SiO₂, MnOand Cr₂O₃, and the respective contents are stipulated to lower themelting points of the ternary system mixed oxide of SiO₂, MnO and Cr₂O₃.In this way, the present invention reduces number of the welding defectsin the electric welded portion, and prevents deterioration of the creepcharacteristics and toughness of the electric welded portion.

In other words, the gist of the present invention resides in thefollowing points.

(1) A boiler steel excellent in electric weldability, containing, interms of wt %:

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%, and

Mn: 0.10 to 2.0%, and limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%,

wherein a weight ratio of Si and Mn ((Si %)/(Mn %)) is from 0.005 to1.5;

the balance Fe and unavoidable impurities; and

a melting point of a mixed oxide of SiO₂ and MnO formed at the time ofelectric welding is not higher than 1,600° C.

(2) A boiler steel excellent in electric weldability, containing, interms of wt %:

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%,

Mn: 0.10 to 2.0%,

Nb: 0.001 to 0.5%,

V: 0.02 to 1.0%,

N: 0.001 to 0.08%,

B: 0.0003 to 0.01%, and

Al: not greater than 0.01%, containing further at least one of thefollowing elements:

Mo: 0.01 to 2.0%, and

W: 0.01 to 3.0%, and limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si and Mn ((Si %)/(Mn %)) is from 0.005 to1.5;

the balance Fe and unavoidable impurities; and

a melting point of a mixed oxide of SiO₂ and MnO formed at the time ofelectric welding is not higher than 1,6000° C.

(3) A boiler steel excellent in electric weldability, containing, interms of wt %:

C: 0.01 to 0.20%;

Si: 0.01 to 1.0%,

Mn: 0.10 to 2.0%, and

Cr: 0.5 to 3.5; and limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si, Mn and Cr ((Si %)/(Mn+Cr %)) is from 0.005to 1.5;

the balance Fe and unavoidable impurities; and

a melting point of a mixed oxide of SiO₂, MnO and Cr₂O₃ formed at thetime of electric welding is not higher than 1,600° C.

(4) A boiler steel excellent in electric weldability, containing, interms of wt %:

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%,

Mn: 0.10 to 2.0%,

Cr: 0.5 to 3.5%,

Nb: 0.001 to 0.5%,

V: 0.02 to 1.0%,

N: 0.001 to 0.08%,

B: 0.0003 to 0.01%, and

Al: not greater than 0.01%; containing further at least one of thefollowing components;

Mo: 0.01 to 2.0%, and

W: 0.01 to 3.0%; and limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si, Mn and Cr ((Si %)/(Mn %+Cr %)) is from0.005 to 1.5;

the balance Fe and unavoidable impurities; and

a melting point of a mixed oxide of SiO₂, MnO and Cr₂O₃ formed at thetime of electric welding is not higher than 1,600° C.

(5) A boiler steel excellent in electric weldability, according to theparagraph (2) or (4), which further contains, in terms of wt %:

Ti: 0.001 to 0.05%.

(6) A boiler steel excellent in electric weldability, according to theparagraph (2) or (4), which further contains at least one of thefollowing elements:

Cu: 0.1 to 2.0%,

Ni: 0.1 to 2.0%, and

Co: 0.1 to 2.0%.

(7) A boiler steel excellent in electric weldability, according to theparagraph (2) or (4), which further contains:

Ti: 0.001 to 0.05%, and at least one of the following elements:

Cu: 0.1 to 2.0%,

Ni: 0.1 to 2.0%, and

Co: 0.1 to 2.0%.

(8) A boiler steel excellent in electric weldability, according to anyof the paragraphs (2) and (4) through (7), which further contains, interms of wt %, 0.001 to 0.2% of at least one of La, Ca, Y, Ce, Zr, Ta,Hf, Re, Pt, Ir, Pd and Sb.

(9) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, containing, in terms of wt %;

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%, and

Mn: 0.10 to 2.0%; and limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si and Mn ((Si %)/(Mn %)) is from 0.005 to1.5;

the balance Fe and unavoidable impurities; and

an area ratio of a binary system mixed oxide of SiO₂ and MnO at electricwelded portions is not greater than 0.1%.

(10) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, containing, in terms of wt %:

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%,

Mn: 0.10 to 2.0%,

Nb: 0.001 to 0.5%,

V: 0.02 to 1.0%,

N: 0.001 to 0.08%,

B: 0.0003 to 0.01%, and

Al: not greater than 0.01%; containing further at least one of thefollowing elements:

Mo: 0.01 to 2.0%, and

W: 0.01 to 3.0%; and limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si and Mn ((Si %)/(Mn %)) is from 0.005 to1.5;

the balance Fe and unavoidable impurities; and

an area ratio of a binary system mixed oxide of SiO₂ and MnO at theelectric welded portions is not greater than 0.1%.

(11) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, containing in terms of wt %:

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%,

Mn: 0.10 to 2.0%, and

Cr: 0.5 to 3.5%; limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si and Mn plus Cr ((Si %)/(Mn %+Cr %)) is from0.005 to 1.5;

the balance Fe and unavoidable impurities; and

an area ratio of a ternary system mixed oxide of SiO₂MnO and Cr₂O₃ atthe electric welded portions is not greater than 0.1%.

(12) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, containing, in terms of wt %:

C: 0.01 to 0.20%,

Si: 0.01 to 1.0%,

Mn: 0.10 to 2.0%,

Cr: 0.5 to 3.5%,

Nb: 0.001 to 0.5%,

V: 0.02 to 1.0%,

N: 0.001 to 0.08%,

B: 0.0003 to 0.01%, and

Al: not greater than 0.01%; containing further at least one of thefollowing elements:

Mo: 0.01 to 2.0%, and

W: 0.01 to 3.0%; limiting the following elements:

P: to not greater than 0.030%,

S: to not greater than 0.010%, and

O: to not greater than 0.020%;

wherein a weight ratio of Si and Mn plus Cr ((Si %)/(Mn %+Cr %)) is from0.005 to 1.5;

the balance Fe and unavoidable impurities; and

an area ratio of a ternary system mixed oxide of SiO₂, MnO and Cr₂O₃ isnot greater than 0.1%.

(13) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, according to the paragraph (10) or (12), which furthercontains, in terms of wt %, the following element as a base materialcomponent:

Ti: 0.001 to 0.05%.

(14) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, according to the paragraph (10) or (12), which furthercontains, in terms of wt %, at least one of the following elements as abase metal component:

Cu: 0.1 to 2.0%,

Ni: 0.1 to 2.0%, and

Co: 0.1 to 2.0%.

(15) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, according to the paragraph (10) or (12) which furthercontains, in terms of wt %, the following element as a base metalcomponent:

Ti: 0.01 to 0.05%, and contains further at least one of the followingelements:

Cu: 0.1 to 2.0%,

Ni: 0.1 to 2.0%, and

Co: 0.1 to 2.0%.

(16) An electric welded boiler steel pipe having fewer defects atelectric welded portions and excellent in creep rupture strength andtoughness, according to any of the paragraphs (10) and (12) to (15),which further contains, in terms of wt %, 0.001 to 0.2% of at least oneof La, Ca, Y, Ce, Zr, Ta, Hf, Re, Pt, Ir, Pd and Sb as a base metalcomponent.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between a welding defect arearatio and Si, Mn and Cr contents.

FIG. 2 is a graph showing the relationship between the welding defectarea ratio and toughness.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be explained in detail.

The feature of the present invention resides in the following point.Particularly, when an ordinary boiler steel and a low Cr ferrite typeboiler steel are electrically welded, the melting point of a binarysystem mixed oxide of SiO₂ and MnO and the melting point of a ternarysystem mixed oxide of SiO₂, MnO and Cr₂O₃, that greatly affect thedefect and properties of the electric welded portion, are controlled bythe relational formula of the addition amounts of Si and Mn, that isstipulated on the basis of the phase diagram of the binary system oxide,and the relational formula of the addition amounts of Si, Mn and Cr,that is stipulated on the basis of the phase diagram of the ternarysystem oxide, so that the welding defect area ratio of the electricwelded portion can be extremely reduced and the deterioration of thecreep characteristics and toughness at the electric welded portions canbe prevented.

The present invention is directed to ordinary boiler steels, low Crferrite type boiler steels and electric welded boiler steel pipes usingthese steels. The reasons why the component compositions of these steelsare stipulated as described above are as follows.

Carbon (C) forms carbides with Cr, Fe, W, Mo, V and Nb and contributesto the improvement of the high-temperature strength. Carbon itselfstabilizes the texture as an austenite-stabilizing element.

The steels according to the present invention are converted to a mixedstructure of ferrite, martensite, bainite and pearlite when the steelsare annealed and tempered. The C content is important for controllingthe balance of these structures.

When the C content is less than 0.01%, the precipitation amount of thecarbides is not sufficient, and the amount of δ-ferrite becomesexcessive great, so that both strength and toughness are deteriorated.When the C content exceeds 0.20%, on the other hand, the carbidesprecipitate excessively. In consequence, the steel is remarkablyhardened, and formability and weldability are deteriorated. Therefore,the C content is limited to 0.01% to 0.20%.

Silicon (Si) is the element that functions as a deoxidizer and alsoimproves the steam oxidation resistance of the steels. When the Sicontent is less than 0.01%, the effect is not sufficient and when itexceeds 1.0%, toughness drops remarkably, and such a Si content is alsodetrimental to the creep rupture strength. Therefore, the Si content islimited to 0.01 to 1.0%.

Manganese (Mn) is the element that is necessary not only for deoxidationbut also for keeping the strength. To obtain a sufficient effect, atleast 0.10% of Mn must be added. When the Mn content exceeds 2.0%, thecreep rupture strength drops in some cases. Therefore, the Mn content islimited to 0.10% to 2.0%.

Chromium (Cr) is an indispensable element for improving the oxidationresistance and the high-temperature corrosion resistance. When the Crcontent is less than 0.5%, these effects cannot be obtained. When the Crcontent exceeds 3.5%, however, toughness, weldability and heatconductivity drop with the result that the advantages of the low Crferrite steel are deteriorated. Therefore, the Cr content is limited to0.5% to 3.5%.

Niobium (Nb) combines with C and N to form fine carbides and nitrides ofNb(C, N), and contributes to the improvement of the creep rupturestrength. Nb forms stable fine precipitates particularly at 625° C. orbelow, and remarkably improves the creep rupture strength. Furthermore,Nb makes the crystal grains fine and is effective for improvingtoughness. However, these effects cannot be obtained when the Nb contentis less than 0.001%. When the Nb content exceeds 0.5%, on the otherhand, the steel becomes extremely hard, and toughness, formability andweldability drop. Therefore, the Nb content is limited to from 0.001% to0.5%.

Vanadium (V) combines with C and N in the same way as Nb, forms finecarbides and nitrides of V(C, N), and contributes to the creep rupturestrength on the high temperature side for a long time. When the Vcontent is less than 0.02%, its effect is not sufficient. When V isadded in an amount exceeding 1.0%, however, the precipitation amount ofV(C, N) becomes excessive, and strength and toughness are deteriorated,on the contrary. Therefore, the V content is limited to from 0.02% to1.0%.

Nitrogen (N) precipitates in the matrix as the solid solution, or thenitrides or carbon nitrides, mainly takes the form of VN, NbN or therespective carbon nitrides, and contributes to both solid solutionhardening and precipitation hardening. In the present invention, Ncombines with Ti to form TiN and combines further with B andprecipitates as BN. These nitrides contribute to the improvement ofcreep rupture strength. When the N content is less than 0.001%, ithardly contributes to strengthening and when it exceeds 0.08%, the dropof the base metal toughness and strength becomes remarkable. Therefore,the N content is limited to 0.001% to 0.08%.

Boron (B) is the element that is added to secure the following effects.Boron co-segregates with C and stabilizes fine carbides (concretely,M₂₃C₆ carbides). When low Cr ferrite steel is heated at a hightemperature for a long time, W and Mo concentrate on the M₂₃C₆ carbideto change this carbide to a coarse M₆C carbide and invite the drop ofcreep rupture strength and toughness. When B is added, however, M₂₃C₆can be stabilized. In consequence, precipitation of the coarse carbideM₆C can be restricted and the drop of creep strength can be limited.When the B content is less than 0.0003%, however, the effect describedabove cannot be obtained. When the B content exceeds 0.01%, on the otherhand, B segregates excessively in the crystal grain boundary, and thecarbides aggregate and becomes coarse in some cases, due toco-segregation with C, with the result that formability, toughness andweldability are remarkably deteriorated. Therefore, the B content islimited to 0.0003% to 0.01%.

Aluminum (Al) is effective as a deoxidizer. However, sincehigh-temperature strength drops particularly when the Al content exceeds0.01%, the Al content is limited to not greater than 0.01%.

Molybdenum (Mo) is the element that has the hardening functions by solidsolution hardening and by precipitation of fine carbides, is effectivefor improving creep rupture strength, and can be contained, whenevernecessary. However, when the Mo content is less than 0.01%, this effectcannot be obtained. When the Mo content exceeds 2.0%, the effect getsinto saturation and moreover, weldability and toughness aredeteriorated. When Mo is added, therefore, the addition amount ispreferably from 0.01% to 2.0%. Incidentally, when Mo and W are added incombination, the strength of the steel can be improved much more thanwhen the elements are added individually and particularly,high-temperature creep rupture strength can be improved.

Tungsten (W) is the element that exhibits hardening operations by solidsolution hardening and by precipitation of fine carbides, and iseffective for improving creep rupture strength. When the W content isless than 0.01%, these effects cannot be obtained. When the W contentexceeds 3.0%, on the other hand, the steel is remarkably hardened withthe drop of toughness, formability and weldability. Therefore, the Wcontent is limited to from 0.01% to 3.0%. Incidentally, when W and Moare added in combination, the strength improving effect of the steelbecomes remarkable, as described above.

Phosphorus (P), sulfur (S) and oxygen (O) mix as impurity elements intothe steel of the present invention. In order to exhibit the effects ofthe present invention, the upper limits of P, S and O are limited to0.030%, 0.010% and 0.020%, respectively, because P and S lower strength,and O precipitates as oxides and lowers toughness.

Titanium (Ti) combines with C and N and forms Ti(C, N). Particularlybecause Ti has strong binding power with N, it is effective for fixingsolid solution N. Though B, too, has the function of fixing solidsolution N as will be described later, its binding form with C isremarkably different from that of Ti. In other words, B is likely tosegregate into carbides containing Fe, Cr and W as the principalcomponents, and when B exists in excess, B promotes, in some cases,aggregation and coarsening of these carbides. In contrast, Ti combinesindividually with C and undergoes composite precipitation as TiN butdoes not allow the further progress of aggregation and coarsening.Therefore, Ti is preferred in that it effectively fixes N and at thesame time, does not affect phase stability of the carbides.

Furthermore, Ti improves hardenability by restricting the solid solutionN amount, and also improves toughness and creep strength. However, theseeffects cannot be obtained when the Ti content is less than 0.001%. Whenthe Ti content exceeds 0.05%, on the other hand, the precipitationamount of Ti(C, N) becomes so great that toughness is remarkablydeteriorated. Therefore, the Ti content is preferably from 0.001% to0.05%.

All of copper (Cu), nickel (Ni) and cobalt (Co) are strong austenitestabilizing elements. They are necessary, and useful, for obtaining thehardened structure or the hardened-tempered structure particularly whenlarge amounts of ferrite stabilizing elements, that is, Cr, W, Mo, Ti,Si, and so forth, are added. At the same time, Cu is useful forimproving the high-temperature corrosion resistance, Ni, for improvingtoughness, and Co, for improving strength. When their contents are notgreater than 0.1%, the effect is not sufficient, and when they are addedin the amounts exceeding 2.0%, embrittlement, resulting fromprecipitation of coarse inter-metallic compounds or segregation into thegrain boundary, is not avoidable. Therefore, the Cu, Ni and Co contentsare limited to 0.1% to 2.0%, respectively.

All of lanthanum (La), calcium (Ca), yttrium (Y), cerium (Ce), zirconium(Zr), tantalum (Ta), hafnium (Hf), rhenium (Re), platinum (Pt), iridium(Ir), palladium (Pd) and antimony (Sb) are added, whenever necessary, tocontrol the forms of the impurity elements (P, S, O) and theirprecipitates (inclusions). When at least one of these elements is addedin the amount of at least 0.001%, the impurities described above can befixed as stable and harmless precipitates, and strength and toughnesscan be improved. When the addition amount is less than 0.001%, theeffect cannot be obtained, and when the amount exceeds 0.2%, the amountof the inclusions increase and toughness is deteriorated, to thecontrary. Therefore, the contents of these elements are limited to from0.001 to 0.2%.

The present invention stipulates the components of the ordinary boilersteels and the low Cr ferrite type boiler steels as described above.Furthermore, to reduce the defects occurring at the electric weldedportions and to improve creep rupture strength and toughness, thepresent invention stipulates the Si and Mn contents as the formationelements of a binary system mixed oxide of SiO₂ and MnO for the ordinaryboiler steels (Si-Mn component system) by the following formula (1), andstipulates also the Si, Mn and Cr contents as the formation elements ofa ternary system mixed oxide of SiO₂, MnO and Cr₂O₃ for the low Crferrite type boiler steels (Si—Mn—low Cr component system) by thefollowing formula (2).

0.005≦(Si %)/(Mn %)≦1.5  (1)

 0.005≦(Si %)/(Mn+Cr %)≦1.5  (2)

where (Si %), (Mn %) and (Cr %) represent the Si, Mn and Cr contents,respectively.

The results of the experiments conducted by the present inventors haverevealed that the binary system mixed oxide of SiO₂ and MnO a exertsgreat influence on the occurrence of the defects in the ordinary boilersteels (Si—Mn component system) while the ternary system mixed oxide ofSiO₂, MnO and Cr₂O₃ does in the low Cr ferrite type boiler steels(Si—Mn-low Cr component system), but when the melting points of thesemixed oxides are lower than 1,600° C., these oxides do not remain as theoxides in the electric welded portions during electric welding, but aremolten and squeezed out as slag components, so that the weld defects ofthe electric welded portions do not occur so easily.

When the phase diagram of these oxides is looked-up, the melting pointof the mixed oxide becomes lower when the SiO₂ content becomes greater,and becomes higher when the MnO and/or Cr₂O₃ content becomes greater. Inview of this fact, the present invention controls the formation of themixed oxides, that greatly affect the defects and properties of theelectric welded portions, by limiting the addition amounts of Si, Mn andCr as the formation elements of SiO₂, MnO and Cr₂O₃, by theaforementioned formula (1) for the ordinary boiler steel and by theformula (2) for the low Cr ferrite type boiler steel.

FIG. 1 shows the relationship between (Si %)/(Mn %) or (Si %)/(Mn %+Cr%) and the welding defect area ratio of the electric welded portion inboth ordinary boiler steel and low Cr ferrite type boiler steel in thesteels according to the present invention in comparison with the steelsaccording to the prior art. FIG. 2 shows the relationship between thetoughness of the electric welded portion and the welding defect arearatio at that time. Here, the welding defect area ratio of the electricwelded portion is calculated by observing the electric welded portion byan optical microscope, measuring the total area of the mixed oxideconsisting mainly of SiO₂ and MnO for the ordinary boiler steel andSiO₂, MnO and Cr₂O₃ for the low Cr ferrite type boiler steel, andcalculating the area ratio per unit area to obtain the welding defectarea ratio. Toughness is measured by collecting a Charpy test specimenin a C direction (circumferential direction) of the electric weldedportion, and conducting the Charpy test at 100° C.

As shown in FIGS. 1 and 2, when the value of (Si %)/(Mn %) or (Si %)/(Mn%+Cr %) represented by the formula (1) or (2) is less than 0.005, theoxide of MnO or/and Cr₂O₃ remains at the electric welded portion andresults in the welding defect. Therefore, creep rupture strength andtoughness of the electric welded portion drop. When the value of theformulas exceeds 1.5, the SiO₂ oxide remains at the electric weldedportion and results in the welding defect. Therefore, creep rupturestrength and toughness of the electric welded portion drop, too.Therefore, the upper and lower limit values of the formula (1) and (2)are limited to 1.5 and 0.005, respectively.

The area ratio of the binary system mixed oxide of SiO₂ and MnO in theelectric welded portion must be not greater than 0.1% in the electricwelded boiler steel pipe using the ordinary boiler steel, and the arearatio of the ternary system mixed oxide of SiO₂, MnO and Cr₂O₃ must benot greater than 0.1% in the case of the electric welded boiler steelpipe using the low Cr ferrite type boiler steel. When the area ratio ofthe binary system mixed oxide or the ternary system mixed oxide exceeds0.1%, the welding defect area ratio of the electric welded portionexceeds 0.1%, and both creep rupture strength and toughness drops.Therefore, the upper limit is limited to 0.1%

EXAMPLES

Steels having the chemical components shown in Tables 1 to 3 were moltenin a 150 kg vacuum melting furnace and the resulting ingots were heatedand hot rolled at 1,050 to 1,300° C. to obtain sheets having thicknessof 3, 5, 10, 15 and 20 mm. All the hot rolling finish temperatures werecontrolled so that they fell within the range of 900 to 1,000° C. Next,solid solution heat treatment was conducted as the heat treatment forall the steels, and a tempering treatment at 780° C. for 1 hour withair-cooling was conducted. The properties of the base metal and electricwelded portion of each steel after the heat treatment were evaluated bythe creep rupture test, the Charpy impact test and the measurement ofthe welding defect area ratio. In this case, the electric welded portionfracture oxide form, etc, did not change before and after the temperingtreatment of each test specimen used for the welding defect area ratiomeasurement.

Incidentally, a tensile test specimen of φ6 mm×GL 30 mm was used for thecreep rupture test in the evaluation test. The creep rupture test wasconducted for 15,000 hours at the longest at 550° C. and 600° C., andthe creep rupture strength at 550° C. and 600° C. for 100,000 hours wascalculated by extrapolation. A 2 mm V-notch test specimen (JIS4 testspecimen) of 10 mm×10 mm×55 mm was used for the Charpy impact test, anda ductile-brittle fracture transition temperature (vTrs) was determined.The welding defect area ratio was measured by an optical microscopeusing the test specimen subjected to the Charpy test at 100° C.

Tables 1 and 2 show the chemical components of the steels according tothe present invention and their evaluation results. Table 3 shows thechemical components of the Comparative Steels and their evaluationresults. It can be understood that the steels (Nos. 1 to 84) of thepresent invention were superior to the Comparative Examples (Nos. 101 to126) in all properties.

In the Comparative Steels Nos. 105, 109, 113, 121 and 125, the steamoxidation resistance of the steels was not sufficient when the Sicontent was less than 0.01%, and when the Si content exceeded 1.0%,toughness dropped remarkably, and such a Si content was detrimental tocreep rupture strength.

In the Comparative Steels Nos. 106, 110, 114, 115, 118, 122 and 126, itwas necessary to add at least 0.10% of Mn to obtain a sufficientstrength, and when the Mn content exceeded 2.0%, creep rupture strengthdropped in some cases.

In the Comparative Steels Nos. 103, 107, 115, 119 and 123, Cr was theindispensable element for improving the oxidation resistant and thehigh-temperature resistance of the low Cr ferrite steel. If the Crcontent was less than 0.5%, these effects could not be obtained. Whenthe Cr content exceeded 3.5%, on the other hand, toughness, weldabilityand heat conductivity became lower, so that the advantages of the low Crferrite steel became smaller.

In the Comparative Steels Nos. 102, 104, 108, 112, 116, 120, 123, 124and 125, when the value (Si %)/(Mn % +Cr %) was less than 0.005%, oxidessuch as MnO and Cr₂O₃ remained at the electric welded portion andresulted in the welding defects. Also, the properties of the weldportion such as strength and toughness got deteriorated. When the value(Si %)/(Mn %+Cr %) exceeded 1.5%, the SiO₂ oxide remained at theelectric welded portion and resulted in the welding defects with theresult that the properties of the weld portion such as strength andtoughness were deteriorated.

In the Comparative Steels Nos. 101, 116, 117, 123, 124 and 126, when theC content was less than 0.01%, precipitation of the carbides becameinsufficient and the amount of δ-ferrite became so great that strengthand toughness were spoiled. When the C content exceeded 0.20%, on theother hand, the carbides precipitated excessively, and the steels werehardened remarkably. In consequence, both formability and weldabilitywere deteriorated.

TABLE 1 Chemical component of present steels (wt %) and Evaluationresult steel No. C Si Mn P S Cr Mo W Nb V Cu Ni Co Ti  1 0.012 0.0140.119 0.023 0.009  2 0.198 0.980 1.946 0.026 0.008  3 0.011 0.015 0.1100.019 0.006 0.522  4 0.189 0.990 1.950 0.007 0.004 3.480  5 0.161 0.7530.100 0.009 0.002 2.378 0.013 0.015 0.025  6 0.111 0.150 0.120 0.0080.007 1.022 0.015 0.300  7 0.032 0.992 0.223 0.025 0.007 0.521 1.4660.314 0.550  8 0.063 0.493 0.790 0.006 0.004 1.429 1.008 0.249 0.217  90.124 0.709 1.263 0.030 0.003 2.964 1.526 0.222 0.102 10 0.195 0.2561.302 0.019 0.010 0.500 0.012 0.194 0.992 11 0.056 0.555 0.316 0.0170.003 3.231 2.523 0.492 0.843 12 0.097 0.432 1.998 0.015 0.007 1.7051.374 3.000 0.001 0.249 13 0.148 0.014 0.286 0.017 0.002 2.262 0.0650.864 0.080 0.512 14 0.189 0.248 1.552 0.011 0.008 3.492 0.486 1.2220.155 0.197 15 0.010 0.047 0.864 0.023 0.003 2.665 2.000 2.792 0.3320.341 16 0.158 0.022 0.109 0.030 0.009 2.964 0.012 0.223 0.024 0.001 170.031 0.860 0.260 0.026 0.006 3.496 1.666 0.193 0.993 0.033 18 0.0620.012 1.256 0.022 0.005 0.502 1.027 0.493 0.342 0.046 19 0.122 0.6510.205 0.023 0.010 1.555 1.592 0.016 0.192 0.009 20 0.191 0.894 1.4400.016 0.003 2.231 1.230 0.088 0.522 0.025 21 0.055 0.112 0.212 0.0070.002 1.429 2.764 0.337 0.243 0.050 22 0.095 0.931 0.223 0.028 0.0020.751 1.026 0.843 0.194 0.843 0.013 23 0.145 0.843 0.614 0.007 0.0022.989 0.064 3.000 0.153 0.103 0.027 24 0.185 0.992 0.234 0.025 0.0090.531 0.444 2.610 0.284 0.216 0.016 25 0.010 0.346 1.992 0.011 0.0050.854 1.992 0.011 0.316 0.555 0.024 26 0.155 0.021 0.106 0.030 0.0052.875 0.012 0.219 0.022 0.10 27 0.031 0.817 0.265 0.011 0.006 3.4861.633 0.189 0.998 0.86 28 0.061 0.011 1.281 0.015 0.008 0.511 1.0060.498 0.315 1.99 29 0.119 0.618 0.209 0.028 0.010 1.508 1.496 0.0020.177 0.54 30 0.194 0.849 1.469 0.026 0.004 2.164 1.156 0.086 0.480 0.1231 0.054 0.106 0.216 0.025 0.003 1.386 2.598 0.330 0.224 0.62 32 0.0930.884 0.227 0.020 0.008 0.728 1.005 0.792 0.190 0.776 1.53 1.23 33 0.1420.801 0.626 0.013 0.006 2.899 0.063 2.994 0.150 0.095 0.35 0.98 34 0.1820.982 0.239 0.019 0.005 0.515 0.435 2.453 0.278 0.199 0.11 1.98 35 0.0100.329 1.992 0.024 0.005 0.828 1.952 0.010 0.310 0.511 2.00 1.52 1.51 360.010 0.020 1.515 0.025 0.003 2.818 0.012 0.214 0.021 1.98 0.001 370.026 0.801 0.301 0.030 0.005 3.493 1.600 0.185 0.999 0.84 0.032 380.126 0.011 0.593 0.015 0.006 0.501 0.986 0.499 0.305 0.11 0.045 390.020 0.606 1.167 0.026 0.006 1.478 1.526 0.002 0.171 1.99 0.009 400.144 0.832 1.989 0.020 0.008 2.121 1.179 0.085 0.466 1.98 0.025 410.021 0.104 0.527 0.018 0.009 1.358 2.650 0.324 0.217 0.11 0.049 weldingdefect steel 550 CRS 600 CRS vTrs area No. B N Al O others SMC MPa MPa °C. ratio %  1 0.016 0.118 155  98 −30 0.058  2 0.008 0.504 153  96 −490.020  3 0.009 0.024 155  98 −30 0.058  4 0.012 0.182 153  96 −48 0.022 5 0.0003 0.051 0.009 0.020 0.304 154  97 −45 0.027  6 0.0050 0.0080.009 0.013 1.250 159 100 −45 0.028  7 0.0100 0.031 0.009 0.013 1.333163 103 −42 0.034  8 0.0051 0.019 0.001 0.009 0.222 159 100 −40 0.037  90.0084 0.014 0.010 0.001 0.168 159 100 −50 0.018 10 0.0024 0.064 0.0030.002 0.142 155  98 −51 0.016 11 0.0016 0.072 0.004 0.004 0.158 157  99−36 0.046 12 0.0031 0.031 0.005 0.008 0.117 160 101 −42 0.034 13 0.00590.026 0.002 0.009 0.005 157  99 −50 0.019 14 0.0004 0.001 0.002 0.0150.049 155  98 −48 0.021 15 0.0066 0.080 0.001 0.013 0.013 164 103 −370.045 16 0.0003 0.053 0.009 0.012 0.007 154  97 −45 0.028 17 0.01000.034 0.008 0.016 0.229 164 103 −42 0.034 18 0.0065 0.016 0.005 0.0130.007 160 101 −42 0.035 19 0.0005 0.015 0.002 0.010 0.370 155  98 −420.035 20 0.0062 0.062 0.003 0.007 0.244 157  99 −54 0.009 21 0.00350.071 0.001 0.015 0.068 158 100 −38 0.042 22 0.0017 0.034 0.010 0.0090.956 158 100 −40 0.038 23 0.0022 0.025 0.009 0.020 0.234 156  99 −460.027 24 0.0081 0.001 0.006 0.010 1.297 160 101 −56 0.007 25 0.00550.080 0.006 0.016 0.122 162 102 −35 0.047 26 0.0003 0.055 0.009 0.0200.007 154  97 −45 0.028 27 0.0098 0.035 0.008 0.013 0.218 164 103 −420.034 28 0.0064 0.016 0.005 0.017 0.006 156  98 −12 0.095 29 0.00050.015 0.002 0.009 0.360 154  97 −33 0.051 30 0.0061 0.064 0.003 0.0160.234 157  99 −54 0.009 31 0.0034 0.073 0.001 0.019 0.066 158 100 −380.042 32 0.0017 0.035 0.010 0.011 0.925 155  98 −22 0.075 33 0.00220.026 0.009 0.007 0.227 157  99 −45 0.027 34 0.0079 0.001 0.006 0.0141.303 160 101 −53 0.011 35 0.0054 0.079 0.006 0.013 0.117 160 101 −130.093 36 0.0003 0.053 0.009 0.015 0.005 155  98 −30 0.057 37 0.00990.034 0.008 0.015 0.211 164 103 −41 0.035 38 0.0062 0.016 0.005 0.0190.010 159 100 −46 0.026 39 0.0005 0.015 0.002 0.008 0.229 152  96  −20.115 40 0.0060 0.063 0.003 0.020 0.203 158 100 −49 0.019 41 0.00340.072 0.001 0.014 0.055 158 100 −34 0.049 SMC: (Si %)/(Mn % + Cr %)value 550 CRS: estimated creep rupture strength at 550° C. for 100,000hrs. 600 CRS: estimated creep rupture strength at 600° C. for 100,000hrs.

TABLE 2 Chemical component of present steels (wt %) and Evaluationresult steel No. C Si Mn P S Cr Mo W Nb V Cu Ni Co Ti 42 0.022 0.9550.222 0.027 0.010 0.610 0.985 0.808 0.186 0.752 1.50 0.29 0.013 43 0.0610.785 1.393 0.021 0.005 2.841 0.061 3.000 0.147 0.092 0.34 1.52 0.026 440.023 0.999 1.779 0.016 0.010 0.505 0.426 2.502 0.273 0.193 1.62 0.830.016 45 0.195 0.322 0.101 0.016 0.006 0.812 1.984 0.011 0.303 0.4950.10 0.94 0.54 0.024 46 0.160 0.751 0.100 0.008 0.001 2.377 0.013 0.0140.024 47 0.031 0.990 0.222 0.024 0.006 0.520 1.466 0.313 0.549 48 0.0620.491 0.789 0.005 0.003 1.428 1.008 0.248 0.216 49 0.123 0.707 1.2620.029 0.002 2.963 1.526 0.221 0.101 50 0.194 0.254 1.301 0.018 0.0090.500 0.012 0.193 0.991 51 0.055 0.553 0.315 0.016 0.002 3.230 2.5230.491 0.842 52 0.096 0.430 1.997 0.014 0.006 1.704 1.374 3.000 0.0010.248 53 0.147 0.012 0.285 0.016 0.001 1.861 0.065 0.864 0.079 0.511 540.188 0.246 1.551 0.010 0.007 3.491 0.486 1.222 0.154 0.196 55 0.0100.045 0.863 0.022 0.002 2.664 2.000 2.792 0.331 0.340 56 0.157 0.0200.108 0.029 0.008 2.963 0.012 0.222 0.023 0.001 57 0.030 0.858 0.2590.025 0.005 3.495 1.666 0.192 0.992 0.033 58 0.061 0.010 1.255 0.0210.004 0.501 1.027 0.492 0.341 0.046 59 0.121 0.649 0.204 0.022 0.0091.554 1.592 0.015 0.191 0.009 60 0.190 0.892 1.439 0.015 0.002 2.2301.230 0.087 0.521 0.025 61 0.054 0.110 0.211 0.006 0.001 1.428 2.7640.336 0.242 0.050 62 0.094 0.929 0.222 0.027 0.001 0.750 1.026 0.8430.193 0.842 0.013 63 0.144 0.841 0.613 0.006 0.001 2.988 0.064 3.0000.152 0.102 0.027 64 0.184 0.990 0.233 0.024 0.008 0.530 0.444 2.6100.283 0.215 0.016 65 0.010 0.344 1.991 0.010 0.004 0.853 1.992 0.0110.315 0.554 0.024 66 0.154 0.019 0.105 0.029 0.004 2.874 0.012 0.2180.021 0.10 67 0.030 0.815 0.264 0.010 0.005 3.485 1.633 0.188 0.997 0.8668 0.060 0.010 1.280 0.014 0.007 0.510 1.006 0.497 0.314 1.99 69 0.1180.616 0.208 0.027 0.009 1.507 1.496 0.001 0.176 0.54 70 0.193 0.8471.468 0.025 0.003 2.163 1.156 0.085 0.479 0.12 71 0.053 0.104 0.2150.024 0.002 1.385 2.598 0.329 0.223 0.62 72 0.092 0.882 0.226 0.0190.007 0.727 1.005 0.792 0.189 0.775 1.53 1.23 73 0.141 0.799 0.625 0.0120.005 2.898 0.063 2.994 0.149 0.094 0.35 0.98 74 0.181 0.980 0.238 0.0180.004 0.514 0.435 2.453 0.277 0.198 0.11 1.98 75 0.010 0.327 1.991 0.0230.004 0.827 1.952 0.010 0.309 0.510 2.00 1.52 1.51 76 0.010 0.018 1.5140.024 0.002 1.817 0.012 0.213 0.020 1.98 0.001 77 0.025 0.799 0.3000.029 0.004 3.492 1.600 0.184 0.998 0.84 0.032 78 0.125 0.010 0.5920.014 0.005 0.500 0.986 0.498 0.304 0.11 0.045 79 0.019 0.604 1.1660.025 0.005 1.477 1.526 0.001 0.170 1.99 0.009 80 0.143 0.830 1.9880.019 0.007 2.120 1.179 0.084 0.465 1.98 0.025 81 0.020 0.102 0.5260.017 0.008 1.357 2.650 0.323 0.216 0.11 0.049 82 0.021 0.953 0.2210.026 0.009 0.609 0.985 0.808 0.185 0.751 1.50 0.29 0.013 83 0.060 0.7831.392 0.020 0.004 2.840 0.061 3.000 0.146 0.091 0.34 1.52 0.026 84 0.0220.997 1.778 0.015 0.009 0.504 0.426 2.502 0.272 0.192 1.62 0.83 0.016 850.194 0.320 0.100 0.015 0.005 0.811 1.984 0.011 0.302 0.494 0.10 0.940.54 0.024 welding defect steel 550 CRS 600 CRS vTrs area No. B N Al Oothers SMC MPa MPa ° C. ratio % 42 0.0016 0.034 0.010 0.016 1.148 158100 −29 0.081 43 0.0021 0.025 0.009 0.013 0.185 158 100 −37 0.043 440.0078 0.001 0.006 0.012 0.437 158 100 −15 0.088 45 0.0053 0.079 0.0060.009 0.353 159 100 −40 0.039 46 0.0003 0.050 0.008 0.019 La = 0.0010.303 156  98 −46 0.025 47 0.0099 0.030 0.008 0.012 Ca = 0.001 1.334 165104 −43 0.032 48 0.0050 0.018 0.000 0.008 Y = 0.002 0.221 161 102 −410.036 49 0.0083 0.013 0.009 0.000 Ce = 0.001 0.167 161 101 −51 0.017 500.0023 0.063 0.002 0.001 Zr = 0.002 0.141 157  99 −52 0.015 51 0.00150.071 0.003 0.003 Ta = 0.001 0.156 159 100 −37 0.044 52 0.0030 0.0300.004 0.007 Hf = 0.001 0.116 162 102 −43 0.033 53 0.0058 0.025 0.0010.008 Re = 0.002 0.006 159 100 −51 0.017 54 0.0003 0.001 0.001 0.014 Pt= 0.002 0.049 157  99 −49 0.020 55 0.0065 0.079 0.000 0.012 Ir = 0.0010.013 166 105 −38 0.043 56 0.0003 0.052 0.008 0.011 Pd = 0.002 0.007 156 98 −46 0.026 57 0.0099 0.033 0.007 0.015 Sb = 0.002 0.229 166 104 −430.032 58 0.0064 0.015 0.004 0.012 La = 0.17 0.006 162 102 −42 0.033 590.0004 0.014 0.001 0.009 Ca = 0.19 0.369 157  99 −42 0.033 60 0.00610.061 0.002 0.006 Y = 0.2 0.243 159 100 −55 0.008 61 0.0034 0.070 0.0000.014 Ce = 0.18 0.067 160 101 −39 0.040 62 0.0016 0.033 0.009 0.008 Zr =0.16 0.958 160 101 −41 0.036 63 0.0021 0.024 0.008 0.019 Ta = 0.15 0.234158 100 −47 0.025 64 0.0080 0.001 0.005 0.009 Hf = 0.18 1.298 162 102−56 0.005 65 0.0054 0.079 0.005 0.015 Re = 0.2 0.121 164 103 −36 0.04566 0.0003 0.054 0.008 0.019 Pt = 0.18 0.006 156  98 −46 0.027 67 0.00970.034 0.007 0.012 Ir = 0.2 0.217 166 104 −43 0.033 68 0.0063 0.015 0.0040.016 Pd = 0.17 0.006 158 100 −12 0.093 69 0.0004 0.014 0.001 0.008 Sb =0.19 0.359 156  98 −34 0.050 70 0.0060 0.063 0.002 0.015 La = 0.05, Ca =0.12 0.233 159 100 −55 0.007 71 0.0033 0.072 0.000 0.018 Y = 0.08, Ce =0.003 0.065 160 101 −39 0.041 72 0.0016 0.034 0.009 0.010 Zr = 0.12, Ta= 0.008 0.925 157  99 −22 0.073 73 0.0021 0.025 0.008 0.006 Hf = 0.009,Re = 0.12 0.227 159 100 −46 0.026 74 0.0078 0.001 0.005 0.013 Pt =0.009, Ir = 0.11 1.304 162 102 −54 0.010 75 0.0053 0.078 0.005 0.012 Pd= 0.005, Sb = 0.1 0.116 162 102 −13 0.091 76 0.0003 0.052 0.008 0.014 Zr= 0.13, Ir = 0.012 0.006 157  99 −31 0.055 77 0.0098 0.033 0.007 0.014Ca = 0.13, Y = 0.005 0.211 166 104 −42 0.033 78 0.0061 0.015 0.004 0.018Ca = 0.18, Ta = 0.18 0.009 161 102 −47 0.024 79 0.0004 0.014 0.001 0.007Re = 0.011, Sb = 0.002 0.229 154  97  −2 0.113 80 0.0059 0.062 0.0020.019 La = 0.005, Ca = 0.12, 0.202 160 101 −50 0.018 Ta = 0.16 81 0.00330.071 0.000 0.013 Y = 0.16, Zr = 0.16, 0.054 160 101 −35 0.047 Ta = 0.1182 0.0015 0.033 0.009 0.015 Ca = 0.16, Zr = 0.11, 1.148 160 101 −290.059 Hf = 0.008 83 0.0020 0.024 0.008 0.012 Ca = 0.008, Ta = 0.16,0.185 160 101 −38 0.042 Pt = 0.013 84 0.0077 0.001 0.005 0.011 La =0.015, Ca = 0.12, 0.437 160 101 −16 0.087 Y = 0.018, Zr = 0.11 85 0.00520.078 0.005 0.008 Ca = 0.005, Zr = 0.2, 0.351 161 101 −41 0.037 Pd =0.005, Sb = 0.008 SMC: (Si %)/(Mn % + Cr %) value 550 CRS: estimatedcreep rupture value at 550° C. for 100,000 hrs. 600 CRS: estimated creeprupture value at 600° C. for 100,000 hrs.

TABLE 3 Chemical components of Comparative Steels (wt %) and evaluationresult steel No. C Si Mn P S Cr Mo W Nb V Cu Ni Co Ti 101 0.006 0.5621.230 0.009 0.009 102 0.053 0.721 0.460 0.012 0.004 103 0.120 0.7770.109 0.008 0.010 3.569 104 0.051 0.986 0.111 0.023 0.003 0.511 1050.161 1.232 0.326 0.009 0.002 2.641 0.013 0.015 0.025 106 0.124 0.7090.061 0.030 0.003 2.964 1.526 0.222 0.102 107 0.010 0.047 0.864 0.0230.003 0.294 2.000 2.792 0.332 0.341 108 0.158 0.013 0.109 0.030 0.0092.964 0.012 0.223 0.024 0.001 109 0.122 0.008 0.205 0.023 0.010 0.6321.592 0.016 0.192 0.009 110 0.152 0.931 2.614 0.028 0.002 0.751 1.0260.843 0.194 0.843 0.013 111 0.155 0.864 0.106 0.030 0.005 3.864 0.0120.219 0.022 0.10 112 0.025 0.984 0.110 0.015 0.008 0.511 1.548 0.4980.942 1.99 113 0.119 1.164 0.209 0.028 0.010 0.613 1.496 0.003 0.1770.54 114 0.064 0.123 0.084 0.025 0.003 3.214 2.222 0.357 0.547 0.62 1150.149 0.884 2.666 0.020 0.008 0.124 1.005 0.792 0.190 0.776 1.53 1.23116 0.864 0.016 1.989 0.013 0.006 3.492 0.236 2.994 0.147 0.321 0.350.98 117 0.202 0.096 1.222 0.019 0.005 1.114 0.197 0.497 0.258 0.0480.11 1.98 118 0.154 0.424 2.222 0.024 0.005 1.097 0.649 0.397 0.4870.095 2.00 1.52 1.51 119 0.010 0.847 1.515 0.025 0.003 3.995 0.012 0.2140.021 1.98 0.001 120 0.195 0.964 0.111 0.015 0.006 0.501 1.517 0.4990.914 0.11 0.025 121 0.020 1.694 1.167 0.026 0.006 0.601 1.526 0.0030.171 1.99 0.009 122 0.068 0.079 0.064 0.020 0.008 2.222 2.487 0.0220.369 1.98 0.005 123 0.261 0.955 0.222 0.027 0.010 0.412 0.985 0.8080.186 0.752 1.50 0.29 0.013 124 0.436 0.016 1.994 0.021 0.005 3.4890.231 3.000 0.144 0.311 0.34 1.52 0.033 125 0.121 0.006 1.980 0.0160.010 1.092 0.193 0.507 0.253 0.047 1.62 0.83 0.018 126 0.218 0.4162.954 0.016 0.006 1.075 1.984 0.405 0.477 0.092 0.10 0.94 0.54 0.046welding defect steel 550 CRS 600 CRS vTrs area No. B N Al O SMC MPa MPa° C. ratio % 101 0.019 0.457 112 60 13 0.780 102 0.016 1.567 102 56 623.472 103 0.012 0.211 123 78 10 0.775 104 0.008 1.585 111 55 20 1.100105 0.0003 0.051 0.009 0.020 0.415 123 77  6 0.495 106 0.0084 0.0140.010 0.001 0.234 127 80 11 0.865 107 0.0066 0.080 0.009 0.013 0.041 13183  0 0.198 108 0.0003 0.053 0.005 0.012 0.004 123 78  6 0.471 1090.0005 0.015 0.008 0.010 0.010 124 78  3 0.207 110 0.0017 0.034 0.0100.009 0.277 126 79  7 0.547 111 0.0003 0.055 0.005 0.020 0.218 123 78  60.446 112 0.0063 0.061 0.009 0.017 1.585 126 80 30 2.472 113 0.00050.015 0.008 0.009 1.416 123 78  6 0.442 114 0.0034 0.031 0.001 0.0190.037 126 80  0 0.123 115 0.0017 0.035 0.010 0.011 0.317 124 78 12 0.928116 0.0008 0.051 0.009 0.007 0.003 120 75 77 5.818 117 0.0016 0.0030.008 0.014 0.041 124 78 10 0.795 118 0.0049 0.079 0.007 0.013 0.128 12578 10 0.981 119 0.0003 0.053 0.005 0.015 0.154 124 78  9 0.678 1200.0062 0.060 0.009 0.019 1.576 128 81 14 1.130 121 0.0005 0.015 0.0080.008 0.958 122 77 52 2.854 122 0.0036 0.009 0.005 0.020 0.035 127 80  00.326 123 0.0016 0.034 0.010 0.016 1.506 124 78 13 0.853 124 0.00080.050 0.009 0.013 0.003 123 78 34 2.670 125 0.0016 0.003 0.008 0.0120.002 122 77 21 1.598 126 0.0048 0.079 0.007 0.009 0.103 127 80  2 0.189SMC: (Si %)/(Mn % + Cr %) value 550 CRS: estimated creep rupturestrength at 550° C. for 100,000 hrs. 600 CRS: estimated creep rupturestrength at 600° C. for 100,000 hrs.

Industrial Applicability

As described above, the present invention can produce a boiler steel,for use in a high-temperature high-pressure environment, that isexcellent in creep rupture strength and electric weldability, and anelectric welded boiler steel pipe having excellent properties of theelectric welded portion. Since these materials are economical materialsthat can be produced at a low cost of production, the present inventionmakes great contributions to the development of the industry.

What is claimed is:
 1. An electric welded boiler steel pipe having fewerdefects at electric welded portions and excellent in creep rupturestrength and toughness, containing, in terms of wt %: C: 0.01 to 0.20w,Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, and Cr: 0.5 to 3.5%; limiting thefollowing elements: P: to not greater than 0.030%, S: to not greaterthan 0.010%, and O: to not greater than 0.020%; wherein a weight ratioof Si, Mn and Cr ((Si %)/(Mn %+Cr %)) is from 0.005 to 1.5; the balanceFe and unavoidable impurities; an area ratio of a ternary system mixedoxide of SiO₂, MnO and Cr₂O₃ at the electric welded portion is notgreater than 0.1%; and the melting point of the mixed oxide of SiO₂, MnOand Cr₂O₃ formed at electric welding is not higher than 1,600° C.
 2. Anelectric welded boiler steel pipe having fewer defects at electricwelded portions and excellent in creep rupture strength and toughness,containing, in terms of wt %: C: 0.01 to 0.20%, Si: 0.01 to 1.01, Mn:0.10 to 2.0%, Cr: 0.5 to 3.5%, Nb: 0.001 to 0.5%, V: 0.02 to 1.0%, N:0.001 to 0.08%, B: 0.0003 to 0.01%, and Al: not greater than 0.01%;containing further at least one of the following elements: Mo: 0.001 to2.0%, and W: 0.01 to 3.0%, and limiting the following elements: P: tonot greater than 0.030%, S: to not greater than 0.010%, an d O: to notgreater than 0.020%; wherein a weight ratio of Si, Mn and Cr ((Si %)/(Mn%+Cr %)) is from 0.005 to 1.5; the balance Fe and unavoidableimpurities; an area ratio of a ternary system mixed oxide of SiO₂, MnOand Cr₂O₃ at the electric welded portion is not greater than 0.1%; andthe melting point of the mixed oxide of SiO₂, MnO and Cr₂O₃ formed atelectric welding is not higher than 1,600° C.
 3. An electric weldedboiler steel pipe having fewer defects and excellent in creep rupturestrength and toughness, according to claim 2, which further contains, interms of wt %: Ti: 0.001 to 0.05%, as a base metal component.
 4. Anelectric welded boiler steel pipe having fewer defects at electricwelded portions and excellent in creep rupture strength and toughness,according to claim 1, which further contains, in terms of wt %, at leastone of the following elements as a base metal component: Cu: 0.1 to2.0%, Ni: 0.1 to 2.0%, and Co: 0.1 to 2.0%.
 5. An electric welded boilersteel pipe having fewer defects at electric welded portions andexcellent in creep rupture strength and toughness, according to claim 1,which further contains, in terms of wt %: Ti: 0.001 to 0.05%, as a basemetal component, and contains further at least one of the followingelements: Cu: 0.1 to 2.0%, Ni: 0.1 to 2.0%, and Co: 0.1 to 2.0%.
 6. Anelectric welded boiler steel pipe having fewer defects and excellent increep rupture strength and toughness, according to any of claims 2 to 5which further contains, in terms of wt %, 0.001 to 0.2% of at least oneof La, Ca, Y, Ce, Zr, Ta, Hf, Re, Pt, Ir, Pd and Sb as a base metalcomponent.